A query plan (or query execution plan) is an ordered set of steps used to access or
modify information in a SQL relational database management system.

There are typically a large number of alternative ways to execute a given query, with
widely varying performance. When a query is submitted to the database, the query optimizer
evaluates some of the different, correct possible plans for executing the query
and returns what it considers the best alternative.

Please take a look at the sample Query Execution Plan above for the following Query.

SELECT  header.[SalesOrderID] ,header.[OrderDate] ,header.[ShipDate] ,details.[ProductID]
,details.[OrderQty] ,details.[UnitPrice] ,header.[CustomerID] FROM  
[Sales].[SalesOrderHeader] AS header JOIN [Sales].[SalesOrderDetail] AS details
ON header.[SalesOrderID] = details.[SalesOrderID] WHERE header.[CustomerID] = 29560;

If your application performamce is poor, this is one of the aspect you have to look at,
identify the time consuming Queries and review the execution plans of them and see
you could find an optimal one.

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